Oankali

The Oankali are an intriguing alien race of natural genetic engineers in Octavia Butler's excellent Xenogenesis trilogy. There are three sexes: male, female, and the seductiveooloi. The four-armed ooloi are able to manipulate genes directly, and after merging with humans, are able to shape shift, an ability gleaned from studying cancer in humans. The oankali are peaceful vegetarians.